Method and arrangement for continuous sterilization of a liquid milk based product

Abstract: A method of sterilizing a liquid milk based product comprises indirect preheating of the product and final heating to a sterilization temperature 140-150°C by direct steam injection. The product is cooled by so called flash-cooling at which steam corresponding to the added amount of steam is evaporated and the product is further cooled by indirect cooling. It is now proposed that the product prior to the flash-cooling is cooled from the sterilization temperature of 140-150°C to a temperature in the range 90-120°C in an indirect, first cooling step after which the product is cooled by flash-cooling to a temperature of 70-85°C. The arrangement for sterilization comprises a balance vessel (1) connected to an indirectly working first heat exchanger (4) for heating of the milk based product. A steam injection nozzle (6) is arranged in a pipeline leading from the heat exchanger (4). After the steam injection nozzle (6) there is a holding section (8) and a vacuum vessel (10) in which the product is cooled by flash-cooling. A further indirectly working heat exchanger (9) for cooling of the product is connected between the holding section (8) and the vacuum vessel (10).

Description: The present invention relates to a method for continuous sterilization of a liquid milk based product and an arrangement for carrying through this method. The product is preheated by indirect heating and is then heated to a sterilization temperature of 140-150°C by means of injection of steam directly into a flow of the product. After sterilization the product is cooled by so called flash-cooling, during which water vapour corresponding to the added amount of steam is evaporated and the product is cooled by indirect cooling. By injecting the steam directly into the product there is obtained a very rapid heating up to sterilization temperature. Continuous processes with direct steam injection in milk, known as UHT-treatment, were presented in the beginning of the 1960's and are described for example in SE 218 503 (GB 937 782). UHT means Ultra High Temperature. As a consequence of the energy crisis in the 1970's indirect processes for UHT treatments were proposed. In the indirect processes a large amount of the added heat energy may be recovered, whereas with the direct processes the heat energy which is added to the milk when the heating media is injected as steam is lost during the flash cooling. The direct heating of the milk to sterilization temperature followed by flashing may be carried through so rapidly that the holding time at sterilization temperature can be only a few seconds but the bacteriological effect obtained is still sufficient. Indirect sterilization is necessarily slower and the time that the milk is kept at a temperature above 120°C is considerably longer. The longer time the milk is kept at such a high temperature the greater the damage to the different components in the milk. A strong heat treatment results in high lactulose values, which is considered to be a measure of the damage to the milk and indicates a lower milk quality. The interest in direct UHT-processes has now increased again partly because some highly resistant bacteria spores have been found in the milk delivered to dairies periodically, but also because it is desirable to achieve not only a sterile but also high quality sterilized products. According to the invention there is now proposed a new way of carrying through the sterilization treatment which gives a better quality of the treated products, i.e. low formation of lactulose, but still a good operation economy for the sterilization. The method according to the invention is mainly characterised in that the milk based product prior to the flash cooling is cooled from a sterilization temperature of 140-150°C by means of an indirect cooling step to a temperature in the interval 90-120°C, after which the product is cooled by flash-cooling to a temperature of 70-85°C. The liquid milk based products may apart from milk with different fat content consist of cream, reconstituted milk, ice cream mix or condensed or evaporated milk. Milk based products are sensitive to mechanical influence at temperatures above 110°C. According to the proposed method the product is cooled in a mechanically gentle way down to a temperature in the range of 90-120°C and the flash cooling, i.e. the evaporation of steam, starts at this lower level. A smaller amount of incrustations of milk based product on the walls of the vacuum vessel is obtained at this lower operation temperature. The different milk components are subjected to less damage and a higher quality milk product is obtained according to the invention. Apart from less damage to the product, the deaeration and deodorization are carried through in a better way according to the invention. If the product is to be heated to a sterilization temperature of 140°C, the steam needed in an indirectly working heat exchanger system is 25-30 kg steam/1000 kg product. In order to heat the product to the same temperature by steam injection and flashing 135 kg steam are needed per 1000 kg product. According to the present invention only 70-75 kg steam are used per 1000 kg product. The water consumption is rather low when indirectly working processes are used. The heating and cooling water may be circulated in closed circuits. In a direct system working with steam the water consumption is high since a high amount of water, 1000 l cooling water/1000 l product, is needed to cool the vessel where the flash-cooling takes place. When using the method according to the invention only one half of this amount is needed since the flash-cooling is carried through at a lower temperature. According to the method of the invention the milk based product is preheated by an indirect heating to a temperature of 90-125°C, preferably to a temperature of 115-120°C, after which the final heating by way of steam injection into the product takes place. Hitherto the preheating has usually been only to ∼80°C. This means that the amount of steam needed to increase the temperature during the stem injection step is reduced. The method according to the invention usually comprises a protein-stabilizing step during the preheating. The product is kept at the desired temperature for 0.5-5 minutes after heating to 80-100°C. According to the method of the invention the sterilized product is suitably homogenized in a step which is performed after the flash-cooling. Usually the steam injection is carried through in a steam injection nozzle through which the steam and product pass in the form of concentric flows. The proposed method is with advantage carried through with the cooling of the product from sterilization temperature in the first indirect cooling step being at a cooling rate which is above 5°C/sec., i.e. the temperature of the product is lowered by at least 5°C per second. If it is suitable the cooling rate may be to 7-11°C per second. An arrangement for continuous sterilization of a liquid milk based product according to the method of the invention comprises a balance vessel or tank for the product. A first indirectly working heat exchanger is connected to the balance tank. This heat exchanger comprises one or several preheating steps. The steam injection nozzle is arranged in a pipeline leading from the heat exchanger and after the heat injection nozzle there is a holding section. The arrangement also comprises a vacuum vessel in which the product is cooled by way of flash cooling and a second indirect heat exchanger for cooling of the product. The arrangement is mainly characterised in that a further indirectly working heat exchanger for cooling of the product after the steam injection is arranged between the holding section and the vacuum vessel. With advantage this further heat exchanger (cooler) consists of a tubular heat exchanger, where the temperature difference between the cooling media, usually water, and the product is kept high. The present invention is described further with reference to the attached drawing which shows an embodiment of the invention chosen as an example only. Also the temperatures which are given on the drawing must be considered as examples. A balance vessel 1 for the product to be treated according to the method of the invention is connected by way of the pipeline 2 to a pump 3 connected to an indirectly working heat exchanger 4 with in this case two preheating steps. When the product has been heated to a suitable temperature, 90°C, it is brought to pass a holding cell 5 for stabilization of the protein. The product is then returned to the heat exchanger for further preheating to 117°C. The product is then brought to pass a steam injection nozzle 6 in which the product is mixed with steam 7 and by that heated to a temperature of 145°C. After a passage through a short holding section 8, the product is cooled by means of water in an indirectly working heat exchanger 9 to a temperature of 111°C. From the heat exchanger 9 the product is led further to a vacuum vessel 10 in which the product is flash-cooled by evaporating as much liquid as was added by the steam. The product leaves the vacuum vessel with a temperature of 80°C and is then cooled in a second indirectly working heat exchanger 11 to a temperature of 25°C. If it is considered suitable a further pressurizing pump may be arranged between the preheating steps. An homogenisator for homogenizing the product may be connected after the vacuum vessel or possibly after the first preheating step.

Claims: Method for continuous sterilization of a liquid milk based product, which is preheated by indirect heating, heated to sterilization temperature 140-150°C by direct injection of steam into the product, cooled by so called flash-cooling, during which steam, preferably corresponding to the added amount of steam, is evaporated, and further cooled by indirect cooling, characterised in that the product prior to the flash-cooling is cooled from said sterilization temperature to a temperature in the range 90-120°C, in an indirect, first cooling step after which the product is cooled by flash-cooling to a temperature of 70-85°C. Method according to claim 1, characterised in that the product is preheated to a temperature of 90-125°C, preferably 115-120°C, after which steam injection takes place. Method according to claim 2, characterised in that the preheating comprises a protein stabilizing step in which the product after heating to a temperature of 80-100°C is kept at this temperature for 0.5-5 minutes. Method according to claim 1, characterised in that the sterilized product undergoes an homogenizing step after the flash-cooling. Method according to any one of the preceding claims, characterised in that the steam injection is performed by a steam injection nozzle producing concentric flows of steam and product. Method according to claim 1, characterised in that the cooling of the product from the sterilization temperature in the first cooling step is at a cooling rate which is at least 5°C/sec. Arrangement for continuous sterilization of a liquid milk based product by the method of claim 1, comprising a first indirectly working heat exchanger (4) for preheating the milk based product in one or more heating steps, a steam injection nozzle (6) arranged in a pipeline leading from the heat exchanger, a holding section (8) after the steam injection nozzle, a vacuum vessel (10) in which the product is cooled by way of flash-cooling, and a second indirectly working heat exchanger (11) for further cooling the product, characterised in that a further indirectly working heat exchanger (9) for cooling of the product is connected between the holding section (8) and the vacuum vessel (10). Arrangement according to claim 7, characterised in that said further heat exchanger (9) consists of a tube heat exchanger.

Inventors: ARPH OLLE, MALMGREN BOZENA, PALM BENGT, RINGSTROM ROLAND, ARPH, OLLE, MALMGREN, BOZENA, PALM, BENGT, RINGSTROM, ROLAND
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 1994
Main classification: A23C3
IPC codes: A23C3, A23L3